Abu Hurayrah (may Allah be pleased with him) says that a man asked the Messenger of Allah (peace and blessings be upon him): Which prayer is most virtuous after the obligatory prayer? He (peace and blessings be upon him) said: "The prayer that is offered in the middle of the night." Then he asked: Which fasting is most virtuous after the fasting of Ramadan? He (peace and blessings be upon him) said: "That of the month which you call Muharram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4661